Polyurethane and polyisocyanurate foams are conventionally prepared by reacting an organic polyisocyanate (including diisocyanate) "A-side" component with a "B-side" polyol premix component containing organic polyol, blowing agent, surfactant, catalyst, and possibly other additives such as flame retardants, antioxidants, and U.V. stabilizers. These A-side and B-side components may be purchased by the end-user in separate containers and stored for later use. Since decomposition of the HCFC blowing agents has been observed in the B-side premixes during storage and during the process of making the foam, HCFC compositions inhibited against such decompositions would be highly desirable. For example, the preferred 141b blowing agent has been observed to decompose during the foam-making process to up to about 1%, depending on the formulation and reaction conditions, of various decomposition products of which by far the predominant product is 1-chloro-1-fluoroethylene ("1131a"). Inhibition of such decomposition is desired both because of toxicity concerns and because the decomposition is accompanied by the formation of equivalent amounts of acid which in turn causes catalyst deactivation.